1. Field of the Invention
The present dielectric resonator filter relates to radio frequency (RF) transmission systems using spectral shaping techniques to meet spectral occupancy requirements. More particularly, the present invention relates to RF signal filters used to suppress an out-of-band portion of a RF signal to be transmitted from a transmitting device.
2. Discussion of the Background
Multi-channel multi-point distribution service (MMDS), multi-point distribution service (MDS), Instructional Television Fixed Service (ITFS), and private operational fixed service (OFS) are various groups of channels that collectively are referred to as "wireless cable". A description of a wireless cable system, including system components, frequency ranges, channel allocations, etc., is provided in co-pending provisional application, U.S. Ser. No. 60/021,271, entitled "MODULAR BROADBAND TRANSMISSION SYSTEM AND METHODS", filed Jul. 5, 1996, the contents of which are incorporated herein by reference. A description of conventional wireless cable transmitters is provided in Chapter 12 of Berkoff, S, et al., "Wireless Cable and SMATV", Baylin Publications, 1992, pp. 237-252, the contents of this book being incorporated herein by reference.
The Federal Communications Commission (FCC) has allocated frequency spectrum in the 2.150 GHz to 2.162 GHz and 2.5 GHz to 2.686 GHz ranges for wireless cable services. Traditionally, these frequency ranges have been used to broadcast television signals in an analog signal format (e.g., National Television System Committee, NTSC format). The FCC places particular spectral occupancy requirements on wireless cable transmitters so as to minimize "out-of-band" emissions that disturb adjacent channels due to harmonics, spurious responses and intermodulation products. In particular, for signals transmitted in an analog format, the FCC requires that the maximum out-of-band power of a wireless cable channel must be attenuated 38 dB relative to a peak visual carrier at the channel edges and constant slope attenuation from this level to 60 dB relative to the peak visual carrier at 1 MHZ below the lower band edge and 0.5 MHZ above the upper band edge. All out of band emissions extending beyond these frequencies must be attenuated 60 dB below the peak visual carrier power. For signals transmitted in a digital format, the FCC requires that 38 dB of attenuation be provided relative to a licensed average power level at the channel edges, constant slope attenuation from that level to 60 dB attenuation at 3 MHZ above the upper and below the lower channel edge, and 60 dB attenuation below the licensed average power level at all other frequencies.
Traditionally, the out-of-band portion for each channel has been suppressed in conventional wireless cable transmitters by relying on a combination of (1) an inherent spectral shape of the analog video signals, (2) channel filtering of each analog video signal before passing the respective signals to a high-power amplifier, and (3) operating a high-power amplifier at the transmitter in a linear range, well below a compression point of the high-power amplifier (which is an expensive solution that requires a large number of amplifiers to provide the requisite output power).
With the recent technological advance of digital video and signal processing techniques, transmitting video signals in a digital format will likely be adopted in the wireless cable industry as the future format standard. The present inventors identified that conventional wireless cable transmitters are not well suited for supporting the emerging digital format. Identified problems include (1) different spectral characteristics of digitally formatted signals as compared with analog formatted signals, (2) increased emphasis on operating a transmitter at a higher power and closer to an amplifier compression point so as to economically provide greater coverage and greater information content per wireless cable channel, and (3) lack of filtering support for a dual-mode transmitter which is configured to transmit both analog and digital signals. In response to the technological evolution in the wireless cable industry, the present inventors identified the need for a filter used at a transmitter site (between the amplifier and a transmit antenna) that suppresses the out-of-band portion of digital signals for each channel to within FCC regulated levels. In order to be a viable commercial product, the inventors determined that each filter for each channel must be able to accommodate 200 W (average power), economical to manufacture, and exhibit a performance that is invariant to temperature fluctuation associated with operating in a high-power transmitter environment.
Most conventional filter structures are configured for use in receive-only systems and cannot handle the high-power wireless cable signals at frequencies above 2 GHz. A related issue, is a lack of temperature compensation features in conventional filters that would prevent the filter response from varying when subject to significant temperature variations resulting from the high power transmitter application. Resonator cavities and other techniques used for shaping RF energy in conventional systems, are subject to varying performances as a function of temperature. In particular, these variations become particularly pronounced at frequencies above 2 GHz where the RF wavelengths are small relative to thermal-induced expansion/concentration movement of mechanical components (e.g., conductive cavity walls). One reason for the varying performance is that the cavities increase in size with increasing temperature, which results in a downward shift in frequency response. Furthermore, impedance disturbances caused by notch filter devices would create linear distortion in the digital signals.
Dielectric resonators have been used in the RF communications industry for signal oscillator applications. A feature that makes a dielectric resonator attractive in oscillator applications is its inherent frequency stability. More recently, dielectric resonators have been used in filtering applications, two examples of which are discussed below.
A first conventional dielectric notch filter, shown in FIG. 1, was disclosed in U.S. Pat. No. 4,862,122. In FIG. 1, a filter 10 includes a coaxial cable transmission line 12 that couples RF energy at frequencies below 1 GHz to various dielectric resonator devices 14, which are spaced 1/4 of a wavelength from one another. The dielectric resonator devices 14 are directly connected to the coaxial transmission line 12 via separate connectors 18.
As shown in FIG. 2, each dielectric resonator device has a separate cylindrical housing 16 which includes a dielectric support 24, a dielectric resonator 26, a tuning disk 20 and a coupling loop 28. Sub-GHz energy from the coaxial transmission line 12 is coupled through the electrical connector 18 and into the housing 16 via the coupling loop 28. The dielectric resonator 26 cooperates with the tuning disk 20 so as to provide a "notch" spectral response for suppressing a particular frequency from the signal passed through the transmission line 12.
As identified by the present inventors, the above described conventional dielectric notch filter would have limited applicability in a wireless cable transmitter application because the dielectric notch filter is (1) configured for low power receive-only filtering operations at sub-GHz frequencies, (2) bulky in construction due to separate housings 16 needed for the resonator device and separate connectors 18, (3) not temperature invariant or free from impedance disturbances, and (4) not guaranteed to provide a symmetric frequency response and group delay.
FIG. 3 shows another conventional filter that was disclosed in U.S. Pat. No. 5,373,270 and described as an improved multi-cavity dielectric filter in which separate dielectric resonators are placed within a single cylindrical housing instead of the individual housings 16 as shown in FIG. 1. A rectangular shaped waveguide 34 is equipped with connectors 36 for receiving and outputting a RF signal in the sub-GHz frequency range. A center conductor 38 is provided within the transmission line 34 to which a coupling loop 40 is provided through an orifice 47 for each of plural cavities 65. Each cavity 65 is defined by isolation plates 44 and has a dielectric resonator 42 secured therein by a support element 46. The support 46 mechanically couples the dielectric resonator 42 to the walls of the cavity 65. Separate tuning slugs 56 are secured to the housing 32 through a nut 69.
As recognized by the present inventors, the above described multi-cavity dielectric filter provides the RF signal to each of the resonant cavities 65 via separate loops 40, which are difficult to manufacture and will not likely support high power transmitter applications at frequencies above 1 GHz. Furthermore, the above-described multi-cavity dielectric filter does not expressly provide temperature compensation or impedance compensation to temperature variation and offset impedance disturbances caused by the respective dielectric resonators 42.